From heightened concern for an environmental problem, a water base paint or a powder paint has recently attracted the attention as low pollution-type paints without using an organic solvent. Even in the case of these paints, metallic decorative property is required in a diverse range of applications as in a conventional organic solvent-based paint, and therefore use of a metal pigment is indispensable. In powder coating, an article to be coated is substantially limited to those containing metal, due to restrictions of electrostatic coating and high-temperature baking. On the other hand, when a metal pigment is used for a water base paint, the metal pigment may react with water in the paint and turn into black or generate a hydrogen gas in some cases, thus causing a problem of poor storage stability of the paint.
The technologies for improving the storage stability of the paint include the followings. Japanese Patent Laying-Open No. 2-120368 (Patent Document 1) proposes that a mixture of an organophosphate ester having an ester residue containing a saturated aliphatic group and containing no unsaturated aliphatic group, and an organophosphate ester having an ester residue containing an unsaturated aliphatic group is used as a pigment component. Japanese Patent Laying-Open No. 6-57171 (Patent Document 2) proposes that the surface of an aluminum flake is coated with a film of molybdic acid. International Publication WO02/031061 Pamphlet (Patent Document 3) proposes an aluminum pigment capable of forming a film derived from peroxymolybdic acid, and also contains at least one amine selected from among an alkylamine, an arylamine, an alkanolamine and an alkoxylamine.
However, the metal pigment obtained by these technologies is excellent in storage stability as a water base paint, but is inferior in chemical resistance when formed into a film. Accordingly, the use is practically limited only to the case where overcoat is applied, and thus there is a problem such as low versatility.
On the other hand, in plastic coating of a cellular phone and a personal computer, since the article has to be ready for actual use after coating just once from the viewpoint of cost, excellent chemical resistance is also required for the film in these applications. In order to solve the problem, Japanese Patent Laying-Open No. 62-081460 (Patent Document 4) proposes a metal powder coated with a copolymer of trimethylolpropane triacrylate and/or trimethylolpropane trimethacrylate and a small amount of acrylic acid and/or methacrylic acid. Japanese Patent Publication No. 01-049746 (Patent Document 5) proposes a resin-coated metal pigment, wherein the surface of the metal pigment is coated by strong adhesion with a highly three-dimensionalized resin made from a radical polymerizable unsaturated carboxylic acid and/or a phosphate mono- or di-ester having a radical polymerizable double bond and a monomer having 3 or more radical polymerizable double bonds, and the alkali resistance is 1.0 or less and the pigment is not substantially agglomerated in a heat resistance stability test. Japanese Patent Laying-Open No. 64-040566 (Patent Document 6) proposes an aluminum flake having microscopically smooth surface, which is uniformly coated with a copolymer obtained by reacting at least two compounds selected from the group consisting of oligomers and monomers having at least one polymerizable double bond. Japanese Patent Laying-Open No. 2005-146111 (Patent Document 7) proposes a resin-coated aluminum pigment wherein the surface of a material aluminum pigment is coated with a copolymer obtained by polymerizing a monomer having a polymerizable double bond, a monomer having a polymerizable double bond and a benzene ring, and (meth)acrylic acid.
Although these technologies satisfy the market needs in the application to an organic solvent-based paint, there remains a problem that water resistance is not sufficient in the application to a water base paint. Commonly, the chemical resistance may not be improved by a surface treatment for imparting water resistance, while the water resistance may not be improved by a surface treatment for imparting chemical resistance. As a result, it is difficult to achieve both of water resistance and chemical resistance at a sufficiently practicable level.
As a surface treatment technology combining water resistance and chemical resistance, International Publication WO96/038506 Pamphlet (Patent Document 8) proposes a resin-coated metal pigment, wherein (A) at least one kind selected from among a radical polymerizable unsaturated carboxylic acid, and/or a phosphate or phosphonate mono- or di-ester having a radical polymerizable double bond, and/or a coupling agent having a radical polymerizable double bond, (B) a monomer having 3 or more radical polymerizable double bonds and (C) a polymerization initiator are used, which is produced by adding the component (A) first to treat the metal pigment, and then gradually adding at least one of the components (B) and (C) to form a polymerized resin layer on the surface. Japanese Patent Laying-Open No. 2000-044835 (Patent Document 9) proposes a resin-coated metal pigment that is prepared by dispersing a metal pigment obtained by a physical vapor deposition film crushing method in an organic solvent, adding (A) at least one kind selected from a radical polymerizable unsaturated carboxylic acid, a phosphate or phosphonate mono- or di-ester having a radical polymerizable double bond and a coupling agent having a radical polymerizable double bond, and further adding (B) a monomer having 3 or more radical polymerizable double bonds and (C) a polymerization initiator, in which at least one of the components (B) and (C) is gradually added, followed by polymerization.
However, in the above patent documents, the water resistance is evaluated by an amount of gas generated as measured at 50° C. for 24 hours using a paint having a pH adjusted to 9.5. Under these conditions, alkali resistance is substantially evaluated, and therefore water resistance is not properly evaluated. In recent years, the pH of a water base paint is commonly adjusted within a range from about 7.5 to 8.8, and water resistance is commonly evaluated by the generation of gas during at least several days.
In other words, the needs for a low-pollution water base metallic paint with high versatility is increasing more and more, but the technology capable of achieving both of water resistance and chemical resistance at a sufficiently practicable level has never been completed.    Patent Document 1: Japanese Patent Laying-Open No. 2-120368    Patent Document 2: Japanese Patent Laying-Open No. 6-57171    Patent Document 3: International Publication WO02/031061 Pamphlet    Patent Document 4: Japanese Patent Laying-Open No. 62-081460    Patent Document 5: Japanese Patent Publication No. 01-049746    Patent Document 6: Japanese Patent Laying-Open No. 64-040566    Patent Document 7: Japanese Patent Laying-Open No. 2005-146111    Patent Document 8: International Publication WO96/038506 Pamphlet    Patent Document 9: Japanese Patent Laying-Open No. 2000-044835